1. Field of the Invention
The present invention relates to a hand-held drive-in tool for driving fastening elements in a workpiece and including a drive-in ram, a guide in which the drive-in ram is displaceable, a driving unit for displacing the drive-in ram and having at least one driving element that displaces the drive-in ram, a device for tensioning the at least one driving element.
2. Description of the Prior Art
In the drive-in tools of the type described above, a mechanical driving spring, which can be tensioned by a tensioning mechanism, serves as a driving source for the drive-in ram. It is advantageous that the mechanical driving spring is inexpensive so that a drive-in tool of this kind can be produced economically. Further, mechanical springs have the advantage over gas springs that consists in that tensioning of the mechanical spring does not lead to increases in temperature as is the case in gas springs, and that a tensioned spring does not lose the stored energy for a long time, whereas in a gas spring, the energy is gradually lost because of leakage.
However, compared to gas springs, mechanical springs have the disadvantage that when tension is released quickly they lose a considerable portion of the energy stored in the spring because this energy must be expended for accelerating the spring own mass. Since the mass of a mechanical spring is much greater than that of a gas spring, this loss is much greater compared to gas springs. Since an impact process which occurs in the drive-in tools considered herein leads to a very quick release of tension in the spring, the phenomenon described above is very noticeable.
A drive-in tool of the type discussed above is disclosed in DE 40 13 022 A1. This drive-in tool has an impact mechanism which can be driven toward a tool muzzle by a spring for impacting a nail to drive the nail in. An adjusting device for returning the impact mechanism into its initial position has an electric motor and a speed reduction mechanism for the electric motor. A rotary movement of the electric motor is transmitted by the speed reduction mechanism and a toothed disk meshing with the latter, to a hammer body of the impact mechanism for transferring the impact mechanism against the force of the spring into the initial position in which the impact mechanism is ready for an impact process.
The known drive-in tool is disadvantageous in that the ram speed cannot exceed 15 to 20 m/s, which is not sufficient for applications requiring a setting energy higher than 10 to 20 J, e.g., for setting in steel or concrete. This is a result of the circumstance described above that the mechanical spring must expend a portion of the stored energy for accelerating the spring own mass so that this portion of energy cannot be used for accelerating the impact device. Attempting to increase the impact speed of the drive-in tool by providing a stronger spring of the same design only increases the spring own mass, which increases the energy lost in accelerating the spring own mass so that no increase in speed is achieved in the end result.
Therefore, it is the object of the present invention to develop a drive-in tool which avoids the disadvantages discussed above and which makes it possible to increase the drive-in speed while retaining a high drive-in energy by employing simple technical means.